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A More Loss-Tolerant RTP Payload Format for MP3 Audio

A More Loss-Tolerant RTP Payload Format for MP3 Audio. <draft-ietf-avt-rtp-mp3-01.txt>. Ross Finlayson LIVE.COM. Existing RTP payload format for MPEG audio (RFC 2250). Packet boundaries are on MPEG frame boundaries Appropriate for MPEG (1 or 2) layer I or II audio

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A More Loss-Tolerant RTP Payload Format for MP3 Audio

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  1. A More Loss-Tolerant RTP Payload Format for MP3 Audio <draft-ietf-avt-rtp-mp3-01.txt> Ross Finlayson LIVE.COM

  2. Existing RTP payload format for MPEG audio (RFC 2250) • Packet boundaries are on MPEG frame boundaries • Appropriate for MPEG (1 or 2) layer I or II audio • because their frames are true “application data units” (ADUs) - can be decoded independently • Less appropriate for layer III audio (“mp3”) • mp3 frames contain a back-pointer to earlier frames • ADU boundaries don’t correspond to frame boundaries • causes noticeable audio glitches when packets are lost

  3. frame frame frame frame ADU ADU ADU Illustration

  4. frame frame frame frame ADU ADU ADU Illustration

  5. frame frame frame frame ADU ADU ADU If a frame gets lost...

  6. frame frame frame frame ADU ADU ADU If a frame gets lost... Data received, but useless Data lost Data received, but useless

  7. Alternative: Send headers+ADUs, not frames ADU ADU ADU

  8. A new RTP payload format for MP3 • Identical to the existing format (RFC 2250), except • packets contain complete ADUs - not frames • uses a different payload type (dynamic) • Data-preserving rearrangement of the original • no redundancy added => no increase in bit rate • However, implementations now need more knowledge of the internals of MP3 frames

  9. Recent additions to this Internet-Draft • How to handle streams with layer I or II frames mixed with layer III • probably an uncommon situation • ‘Pseudo-code’ for converting between regular “MP3 frames” and “ADU frames”

  10. Notes • For more information - including audio illustrations: • http://www.live.com/rtp-mp3/ • Some multicast MP3 server and receiver tools that (optionally) use this payload format: • http://www.live.com/liveCaster/ • Let’s have some more implementations!

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